This invention relates to particulate solid flow through a hopper and in particular to the control of such flow.
Fluidized bed combustors are recognized as a means of burning particulate fuel to generate heat. Typically, fuel such as coal having a maximum size of about 3 to 6 millimeters along with a small amount of similarly sized noncombustible particulate material is fed to a fluidized bed. Such beds operate in the temperature range of about 750.degree. to 925.degree. C. The particulate material comprising this bed contains in addition to the fuel, a sulfur absorbent such as limestone and ash from previously burned fuel.
The noncombustible particulate material tends to accumulate within the bed and must be drained from the bed, while maintaining an appropriate bed height of the remaining material. Drainpipes normally extend through the bed support plate into the bed to receive such particulate material. The drained material passes to a conveying means located beneath the fluidized bed combustor. A pneumatic transport line is often used to convey the solids away.
Because of the high temperature occurring within the bed, the material drained from the bed is hot and must be cooled to avoid damage to mechanical equipment and hazard in the disposal operation. Cooling of the solids generally dictates a spreading of the solids to a substantial flow area, compared to the area where they leave the bed, before combining the solids to a small flow area for introduction into the transport line. This increase in flow area allows particles to contact an appropriate surface of a cooler.
Occasional large particles may at times be encountered. Typical hopper flow problems and pluggage therefore occur.
The transport line has a pressure in the order of 40 to 80 KPa (6 to 12 psi) where it receives the solids compared to 14 KPa (2 psi) pressure within the fluidized bed itself. Some means must be provided to prevent or restrict air flow backwardly from the transport line into the bed. Rotary valves are expensive and subject to substantial wear in the ash laden atmosphere. Lock hoppers use substantial height, require cycling of the valves with concomitant wear, and result in erratic control.
A standing leg of the ash particles was proposed to act as an air seal against flow from the transport line. The solids would flow downward in the leg, with the solids flow controlled by a slide gate valve at the bottom of the leg.
An attempt was made to drain the solids in such manner into a conical hopper with a gate valve controlling the flow from the hopper. With a large valve in the order of 20 cm opening, I found flow occurring with low pressure in the transport line, but stoppage of the flow occurring against a high back pressure. In using a small valve in the order of 5 cm opening, I found the flow intermittently stopping even with low back pressure.